lkdtm/bugs: Do not confuse the clang/objtool with busy wait loop

Since commit eb972ea ("lkdtm/bugs: Add cases for BUG and PANIC
occurring in hardirq context"), building with clang for x86_64 results
in the following warnings:

vmlinux.o: warning: objtool: lkdtm_PANIC_IN_HARDIRQ(): unexpected end of section .text.lkdtm_PANIC_IN_HARDIRQ
vmlinux.o: warning: objtool: lkdtm_BUG_IN_HARDIRQ(): unexpected end of section .text.lkdtm_BUG_IN_HARDIRQ

caused by busy "while (wait_for_...);" loops. Add READ_ONCE() and
cpu_relax() to better indicate the intention and avoid any unwanted
compiler optimisations.

Fixes: eb972ea ("lkdtm/bugs: Add cases for BUG and PANIC occurring in hardirq context")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512190111.jxFSqxUH-lkp@intel.com/
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: ctmarinas

drivers/misc/lkdtm/bugs.c

@@ -120,8 +120,8 @@ static void lkdtm_PANIC_IN_HARDIRQ(void)
 			       CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	hrtimer_start(&timer, us_to_ktime(100), HRTIMER_MODE_REL_HARD);
 
-	while (wait_for_panic)
-		;
+	while (READ_ONCE(wait_for_panic))
+		cpu_relax();
 
 	hrtimer_cancel(&timer);
 }
@@ -150,8 +150,8 @@ static void lkdtm_BUG_IN_HARDIRQ(void)
 			       CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	hrtimer_start(&timer, us_to_ktime(100), HRTIMER_MODE_REL_HARD);
 
-	while (wait_for_bug)
-		;
+	while (READ_ONCE(wait_for_bug))
+		cpu_relax();
 
 	hrtimer_cancel(&timer);
 }